How to model concavity, depression on a mattress surface?
This is really just a starting suggestion; it could be tweaked for ever..
Prepare your mattress for cloth simulation.. I have subdivided this box, triangulated its geometry, and decimated, for this random triangulation. I'm not claiming this is necessarily the best for this. Other topologies may work better.
Create your pattern as flat, reasonably dense, geometry, and Knife Project it down onto the mattress. It will remain selected. Make a Vertex group from the projection...

..and use it as a Pin group in a cloth simulation, including an inflation pressure, which the group will resist.

I'm sure this could be refined further. Whether or not this is better than, say, masked sculpting, is another question. Maybe someone who knows more about that will contribute an answer.